A colloid is a suspension of small particles suspended in a fluid. Although the fluid may be a gas or a liquid, the present invention relates to suspensions of particles in a liquid. The particles may be solid, or they may be emulsion droplets. For more details of the properties of colloids, the reader is referred to the text "Foundations of Colloid Science" by R J Hunter (Oxford Press 1987).
Colloidal particles nearly all carry an electric charge. The charge, which usually resides on the particle surface, can arise from a number of mechanisms including dissociation of acidic or basic groups on the surface or by the adsorption of ions from the surrounding liquid. This charge is balanced by an equal and opposite charge in the liquid. These opposite charges form a diffuse cloud around the particle. The voltage drop between the particle surface and the liquid outside this cloud is called the "zeta potential", denoted by the symbol .zeta.. The invention described is intended to measure the .zeta. potential and from that quantity the surface charge can be calculated (see e.g chapter 2 of "Zeta Potential in Colloid Science" by R. J. Hunter, Academic Press, 1981).
Colloids have many applications in industries ranging from coatings and ceramics to pharmaceuticals and food processing. In many of these applications the efficiency of the process or the quality of the end-product depends crucially on the .zeta. potential and size of the colloidal particles.
Known methods for determining particle size include electron microscopes, Coulter Counters, centrifuges and dynamic light scattering devices. Each of these devices suffers from the drawback that they can only deal with suspensions in which the particles are of a uniform composition. Thus they would not be able to determine the individual size distribution within a colloid of a mixture of titania and silica particles for example.
These techniques also suffer from the drawback that they do not determine the .zeta. potential of the particles.
In U.S. Pat. No. 5,059,909 "Determination of Particle Size and Charge" to the present applicant there is described a method for determining both size and .zeta. potential from "electroacoustic" measurements, that is measurements of the sound waves generated by applied electric fields or measurements of the voltages and electric currents generated by applied sound waves in the colloid. In the analysis provided in this patent it is assumed that the suspension is macroscopically homogeneous, and this technique is limited to particles of uniform composition. It is also implicitly assumed that the particles all have the same .zeta. potential. The technique disclosed is limited to the use of at least 2 different frequencies in the applied fields. The phenomenon of sound wave generation by an applied electric field is called the Electrokinetic Sonic Amplitude (ESA) effect and is described in U.S. Pat. No. 4,497,208 to Oja et al. The reverse effect of electric field generated by an applied sound wave is termed the "Colloid Vibration Potential" or CVP, and has been described in the scientific literature (see for example the review article "Ultrasonic vibration potentials" Zana R. and Yaeger E. B. in Modern Aspects of Electrochemistry, vol 14. Plenum). All the techniques described above are limited to particles of uniform composition.
It is an object of the present invention to provide a method and device capable of measuring both size and charge distribution in at least dilute mixed particle colloids.